One of the steps of sensor fabrication when developing device systems such as are used for infrared (IR) cameras involves the use of a conducting paste to make connections between interconnects of the device. Historically, this junction of the connections between five micron device level interconnects, such as the sensor and the processor, has created a problem. Current commercially available paste makes it necessary to load a higher metal for sintering in these pastes, but a high loading system has no mechanical strength. However, even when a low metal loading system is used, no sintering occurs even at very high temperatures (e.g., 300° C.). Therefore, it is advantageous to have a conducting paste that shows a good electrical yield as well as low contact resistance in order to allow adhesion between the sensor and the processor of, for example, infrared cameras and the like while avoiding the aforementioned problems. This can be achieved by using a solvent free paste, such as described hereinbelow in the present invention, which paste can be cured and completely sintered at or below 200° C.
The existing prior art tends to deal with conducting paste that has the same problems as the commercially available paste listed above. None of the prior art discloses creating a conducting paste for interconnects that includes the same materials and method to connect micron interconnects such as sensors or chips. This again can be solved with a solvent free paste, such as disclosed in the present invention.
U.S. Pat. No. 6,669,079 for CONDUCTIVE PASTE AND SEMICONDUCTOR COMPONENT HAVING CONDUCTIVE BUMPS MADE FROM THE CONDUCTIVE PASTE by Li, et al. discloses a conductive paste to make conductive bumps on a substrate. The conductive paste is formed by combining a tin alloy with a flux composition containing an aromatic carboxylic acid fluxing agent and a solvent. The conductive paste is disposed on underbump metallization layers and reflowed to form the conductive bumps. This conductive paste uses a tin alloy and does not connect interconnects on a micro scale level. In addition, the paste is not made by mixing two different conducting pastes, where each paste maintains its own micro level individual rich region in the mixed paste even after final curing.
United States Patent Application No. 2009/0162557 published Jun. 25, 2009 for NANOSCALE METAL PASTE FOR INTERCONNECT AND METHOD OF USE by Lu, et al. discloses a paste including metal or metal alloy particles (which are preferably silver or silver alloy), a dispersant material, and a binder to form an electrical, mechanical or thermal interconnect between a device and a substrate. By using nanoscale particles (i.e., those which are less than 500 nm in size and most preferably less than 100 nm in size), the metal or metal alloy particles can be sintered at a low temperature to form a layer that allows good electrical, thermal and mechanical bonding. The metal or metal alloy layer can enable usage at a high temperature such as would be desired for SiC, GaN, or diamond wide bandgap devices.
Furthermore, significant application of pressure to form the densified layers is not required, as would be the case with micrometer-sized particles. In addition, the binder can be varied so as to insulate the metal particles until a desired sintering temperature is reached, thereby permitting fast and complete sintering. Unlike the present invention, the '557 reference requires the presence of a dispersant in sufficient quantity to reduce or prevent agglomeration of the particles and a binder having a temperature of volatilization below the sintering temperature of the metal or metal alloy powder.
United States Patent Application No. 2005/0093164 published May 5, 2005 for PASTE FOR FORMING AN INTERCONNECT AND INTERCONNECT FORMED FROM THE PASTE by Maria Standing discloses a paste that includes a mixture of binder particles, filler particles and flux material. The binder particles have a melting temperature lower than that of the filler particles. The proportion of the binder particles and the filler particles is selected so that, when heat is applied to melt the binder particles, the shape of the paste as deposited is substantially retained thereby, allowing for the paste to be used for forming interconnect structures. The present invention does not contain binder particles or any parts that include tin-silver solder, such as prior art binder materials that contain 95.5% Sn, 3.8% Silver, and 0.7% Cu by weight.
The prior art listed above all deals with methods of forming electrically conducting paste, but none of the pastes or methods is similar to the present invention that forms a solvent free paste that can be cured and completely sinters at or below 200° C.
When performing the process of joining the connections between five micron device level interconnects, a conducting paste of choice is one that has a good electrical yield and a low contact resistance, such as the present invention.